<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <script type="text/javascript">

        function f1(){
            //alert("Hello World");

            var i=100;

            console.log(i);

            console.log(typeof i);

            i=false;

            console.log(i);

            console.log(typeof i);

            i='abc';
            i="abc";

            console.log(i);

            console.log(typeof i);

            i=null;

            console.log(i);

            console.log(typeof i);

            var a;

            console.log(a);

            console.log(typeof a);


        }

        /*
            undefiend   结果为false
            string      空字符串为false，反之为true
            number      0为false，反之为true
            object      null为false，反之为true
         */
        function f2(){
            var i=null;
            if(i){
                alert(true);
            }else{
                alert(false);
            }
        }

        function f3(){
            var s1="123";
            var s2='abc';

            console.log(s1+s2);

            console.log(s1+1);

            //string->number
            var i=parseInt(s1);

            console.log(i+1);

            console.log(typeof i);

            var s3="123abc";

            console.log(parseInt(s3));

            var s4="abc123";

            console.log(parseInt(s4));//NaN  Not a Number

            console.log(typeof parseInt(s4));

            var n=1/0;

            console.log(n);//Infinity   无穷大

            console.log(typeof n);

            console.log(n>56);

            var s5="12.345";

            console.log(parseInt(s5));

            console.log(parseFloat(s5));

            console.log(12.345-12);

            console.log(2.6-2);

            //number->string
            console.log(123+"");

        }


    </script>

</head>
<body>
    <input type="button" value="test1" onclick="f1();"/>
    <input type="button" value="test2" onclick="f2();"/>
    <input type="button" value="test3" onclick="f3();"/>
</body>
</html>